1. Field of the Invention
The invention relates to polishing of recording elements such as floppy discs to eliminate recording layer defects by providing intimate contact between the recording material and the magnetic head.
2. Description of the Prior Art
Floppy discs are flexible annular recording discs which are used for digital data storage in data processing systems. Typically, a floppy disc (also commonly referred to as a "flexible disc") includes an annular mylar substrate coated on both sides with a very thin magnetic oxide layer including many minute magnetic oxide particles dispersed in a resin binder. Various techniques for polishing or burnishing the magnetic oxide to remove protruding particles of iron oxide or lumps of binder material formed in the manufacture of the magnetic layer. (The term "polishing" is the technically correct term for describing the above operation of removing protruding particles of iron oxide etc. However, the term "burnishing" is the term ordinarily utilized by those skilled in the art, and is the term hereinafter used in the descriptive portion of this patent application to mean "polishing". The technically correct term "polishing" is utilized in the claims.) If the protruding particles are not removed or smoothed, the magnetic heads (which record and sense digital information into and out of the magnetic oxide layer, also referred to as the magnetic recording layer) are raised slightly as they pass over such particles. It is known that the accuracy or fidelity of both recording and sensing of information in the magnetic recording layer depends upon very "intimate" or close contact between the magnetic head and the magnetic recording layer surface. If such intimate contact is momentarily lost (because the magnetic head is raised slightly as it passes over a protruding particle of the magnetic recording layer), digital information may be "lost".
The state of the art of burnishing flexible recording elements, such as floppy discs, to remove or smoothen protruding particles and high spots from their recording surfaces is exemplified by U.S. Pat. No. 3,943,666, assigned to Dysan Corporation, and by the article, Magnetic Disc Treatment, IBM Technical Disclosure Bulletin, Volume 14, No. 5, Oct. 1971.
The known techniques for burnishing a floppy disc involve placement of the floppy disc on a resilient surface of a rotating platen and bringing a hard abrasive member such as a ceramic disc or blade in contact with the recording surface of the rotating floppy disc, thereby abrading and smoothening an annular region of the magnetic oxide layer. However, such techniques produce non-uniform removal of material from the annular region of the recording layer, due to the variations in relative velocities between the abrasive member and the inner and outer portions of the annular region. Consequently, the inner portions of the annular region may be underburnished, or the outer portions may be overburnished. The manufacturing yield is thereby reduced, increasing the final cost of the floppy discs.
The cost of the floppy discs is further increased because ceramic abrasive elements are expensive and their useful life is quite short because the removed magnetic oxide material tends to "build up" in the ceramic abrasive material, and also wears-out the ceramic material. Further, "wipers" may in some uses be required to remove debris consisting of abraded magnetic oxide particles from the recording surface to prevent such particles from contributing to scratching of the recording layer during the continuing passing of the recording surface under the abrasive member. Such wipers add complexity and expense to the burnishing process and the burnishing machinery.
It should be noted that the floppy disc industry is a very competitive industry, and cost savings of less than 1/2 cent per floppy disc are therefore very important. It is believed that the typical cost of the polishing operation for prior art floppy disc burnishing techniques is of the order of 10 cents per floppy disc.
Thus, there is a need for a means for producing floppy discs which are more uniformly burnished than presently available floppy discs. There is also a need for a technique and apparatus for burnishing floppy discs at much lower cost than presently available techniques and apparatus.